Adding ion sensitivity to an integrated computational element (ICE)

ABSTRACT

A device including an ion-selective membrane arranged within an optical path of the device and coupled to a sample cell to interact with a fluid sample and thereby modify an optical response of the ion-selective membrane according to an ion concentration in the fluid sample, is provided. The device also includes an integrated computational element (ICE) arranged within the optical path, so that the illumination light optically interacts with the ICE and with the ion-selective membrane to provide a modified light that has a property indicative of the ion concentration in the fluid sample. A detector that receives the modified light provides an electrical signal proportional to the property of the modified light. A method and a system for using the above device are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.15/777,170, filed May 17, 2018, which claims the benefit of and is theNational Stage application of International Application No.PCT/US2016/015182, filed Jan. 27, 2016, the disclosures of which areincorporated by reference herein in its entirety.

BACKGROUND

In the field of oil and gas exploration and extraction, measurement ofion concentration in fluids is performed via complex sampling techniquesinvolving chemical reagents and time-consuming procedures. However, thehigh potential for error and inaccuracies, and the low time resolutionof traditional ion measurement techniques, can be impractical in manydownhole situations where conditions may change rapidly under harshenvironments.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of thepresent disclosure, and should not be viewed as exclusive embodiments.The subject matter disclosed is capable of considerable modifications,alterations, combinations, and equivalents in form and function, as willoccur to those skilled in the art and having the benefit of thisdisclosure.

FIG. 1A illustrates a system for measuring an ion concentration of asample fluid using an optical computing device including an integratedcomputational element (ICE).

FIG. 1B illustrates a system for measuring an ion concentration of asample fluid using an optical computing device.

FIG. 2 illustrates a cross-sectional view of an exemplary integratedcomputational element (ICE) for measuring an ion concentration of asample fluid.

FIG. 3 illustrates spectra of a sample light interacted with anion-selective membrane when the sample fluid includes three differention concentrations.

FIG. 4 illustrates a system for measuring a plurality of ionconcentrations of a sample fluid using an optical computing device.

FIG. 5A illustrates a system for measuring an ion concentration of asample fluid using an optical computing device that includes an opticalwaveguide.

FIG. 5B illustrates an optical waveguide including an ion-selectivemembrane for use in an optical computing device for measuring an ionconcentration of a sample fluid.

FIG. 6 illustrates a system for measuring an ion concentration of asample fluid using an ion-selective membrane that interacts with asample in a first point and interacts with an illumination light in asecond point.

FIG. 7 illustrates a wireline system configured to measure an ionconcentration of a sample fluid during formation testing and samplingwith an optical computing device.

FIG. 8 illustrates a field deployment of a fluid analysis systemincluding multiple optical computing devices for measuring an ionconcentration of a sample fluid coupled through an optical fiber link.

FIG. 9 illustrates a flow chart including steps in a method formeasuring an ion concentration of a sample fluid.

In the figures, elements or steps having the same or similar referencenumerals have the same or similar description and configuration, unlessstated otherwise.

DETAILED DESCRIPTION

The present disclosure relates to systems, devices, and methods formeasuring ion concentrations in sample fluids in the oil and gasexploration and extraction industry. Water is often a by-product ofextracted hydrocarbon fluid, and it is desirable to determine where theextracted water comes from within a wellbore. More particularly, wateror a water-based solution is often injected into a wellbore as part ofthe hydrocarbon extraction operation in order to retrieve hydrocarbonsor to introduce additives to facilitate drilling and extractionoperations. It can be beneficial to establish whether water or awater-based solution produced from a wellbore constitutes wateroriginating from the well or water deliberately injected into the well.

For example, in some embodiments an additive may be injected into thewellbore with the objective of sealing, repairing, or modifying in someway the hydrocarbon production in certain parts of an extraction well,such as a production zone. Such additives may include, for example,cement. In such situations, it is desirable to know whether the cementis settling in the intended location. Moreover, water-based solutionsincluding different ions may improve the mobility of the oil and therebyits extraction efficiency from the wellbore. Accordingly, different ionsolutions are often injected into subterranean formations surroundingthe wellbore to facilitate oil extraction. In such configurations, it isdesirable to measure which ion arrives first to the wellbore todetermine which ion solution worked more efficiently in increasing oilmobility in the surrounding formations. Some production operationsproceed by injecting a known ion concentration in the additive fluid,which, upon being measured at the wellbore, provides positive indicationof the location of the settlement of the additive fluid within thesurrounding formations.

Current downhole measurement techniques tend to address physicalproperties of samples and fluids, such as mechanical properties,electrical properties, or the chemical identity of certain compounds.Current wellbore wireline technology is often used to analyze thechemical makeup of downhole fluids and can determine the presence andquantity of several molecules or groups of chemicals such as CO₂,asphaltenes, methane, ethane, propane, water, saturates, aromatics,resins, gas/oil ratios (GOR), and H₂S. However, in order to understandthe chemical status of a downhole operation such as pH, reactivity,and/or chemical stability, a wet chemical analysis is typicallyperformed at the well surface. Embodiments of the present disclosureprovide the capability of undertaking wet chemical analysis downholewithin a wellbore in real time.

Embodiments described in the present disclosure extend the sensingadvantages of optical computing devices to individual ion species suchas, but not limited to, Na⁺, Ca⁺² K⁺, Mg⁺², Cr⁺³, HCO₃ ⁻, SO⁻, NH⁺, andNO₃ ⁻. Embodiments described herein may facilitate measurement ofpositive ions or negative ions in such ion species depending on thedesired target. Optical computing devices as disclosed herein combineintegrated computational element (ICE) technology and ion-selectiveoptode technology to monitor in real time ion concentrations of downholefluids, products, and various chemical and physical propertiesassociated therewith.

An ICE as disclosed herein is an element or device that opticallyinteracts with a substance to determine quantitative and/or qualitativevalues of one or more physical or chemical properties of the substance.The ICE may include multilayered interference elements designed tooperate over a continuum of wavelengths in the electromagnetic spectrumfrom the ultraviolet (UV, about 290 nm to about 400 nm) to mid-infrared(MIR, about 2500 nm to about 10,000 nm) ranges, or any sub-set of thoseregions. Electromagnetic radiation that optically interacts with the ICEis modified to be readable by a detector such that an output of thedetector can be correlated to the physical or chemical property or“characteristic” of the substance being analyzed.

As used herein, the term “characteristic” refers to a chemical,mechanical, or physical property of a substance or a sample of asubstance. A characteristic of a substance may include a quantitative orqualitative value of one or more chemical constituents or compoundspresent therein, or any physical property associated therewith. Suchchemical constituents and compounds may be alternately referred toherein as “analytes.” Illustrative characteristics of a substance thatcan be monitored with the optical computing devices described herein caninclude chemical composition (e.g., identity and concentration in totalor of individual components), phase presence (e.g., gas, oil, water,etc.), impurity content, ion content, pH, alkalinity, viscosity,density, ionic strength, total dissolved solids, salt content (e.g.,salinity), porosity, opacity, bacteria content, total hardness,combinations thereof, state of matter (solid, liquid, gas, emulsion,mixtures, etc.), and the like.

As used herein, the term “electromagnetic radiation” refers to radiowaves, microwave radiation, mid-infrared (MIR) and near-infraredradiation (NIR), visible light (VIS), ultraviolet light (UV), X-rayradiation and gamma ray radiation.

As used herein, the term “optical computing device” refers to an opticaldevice that is configured to receive an input of electromagneticradiation, to interact the electromagnetic radiation with a substanceand to produce an output of electromagnetic radiation from a processingelement arranged within the optical computing device. In someembodiments, an optical computing device also includes a detector togenerate an electronic signal indicative of a characteristic of thesubstance. The processing element may be, for example, an ICE, which isalternately referred to as a multivariate optical element (MOE). Theelectromagnetic radiation that optically interacts with the processingelement is modified so as to be readable by a detector, such that anoutput of the detector can be correlated to a particular characteristicof the substance. The output of electromagnetic radiation from theprocessing element can be reflected, transmitted, and/or dispersedelectromagnetic radiation. Whether the detector analyzes reflected,transmitted, or dispersed electromagnetic radiation may be dictated bythe structural parameters of the optical computing device as well asother considerations known to those skilled in the art. In addition,emission and/or scattering of the fluid, for example via fluorescence,luminescence, Raman, Mie, and/or Raleigh scattering, can also bemonitored by optical computing devices.

As used herein, the term “optically interact” or variations thereofrefers to the reflection, transmission, scattering, diffraction, orabsorption of electromagnetic radiation either on, through or from oneor more processing elements (i.e., ICE or MOE components) or a substancebeing analyzed by the processing elements. Accordingly, opticallyinteracted light refers to electromagnetic radiation that has beenreflected, transmitted, scattered, diffracted, or absorbed, emitted, orre-radiated, for example, using a processing element, but may also applyto interaction with a substance.

In a first embodiment, a device includes an ion-selective membranearranged within an optical path of the device and coupled to a samplecell to interact with a fluid sample and thereby modify an opticalresponse of the ion-selective membrane according to an ion concentrationin the fluid sample, wherein the optical path is defined by anillumination light. The device also includes an integrated computationalelement (ICE) arranged within the optical path, wherein the illuminationlight optically interacts with the ICE and with the ion-selectivemembrane to provide a modified light that has a property indicative ofthe ion concentration in the fluid sample. A detector receives themodified light and provides an electrical signal proportional to theproperty of the modified light.

In a second embodiment, a device includes an optical waveguide thattransmits an illumination light to a detector. The optical waveguidecouples with a sample in at least one sample portion of the waveguide.The device also includes an ion-selective membrane arranged within thesample portion of the waveguide to interact with the sample and therebymodify an optical response of the ion-selective membrane according to anion concentration in the sample. The device also includes an integratedcomputational element (ICE) arranged within an optical path defined bythe illumination light to provide a modified light that has a propertyindicative of the ion concentration in the sample. In some embodiments,the detector receives the modified light and provides an electricalsignal proportional to the property of the modified light.

In yet another embodiment, a method includes exchanging ions exchangebetween a material and a sample and optically interacting the materialwith an illumination light generated by an optical source to generatemodified light. The method further includes detecting a change in theoptical response of the material based on the modified light anddetermining an ion concentration in the sample from the change in theoptical response of the material. The method may further includemodifying an operational parameter in a wellbore for oil and gasexploration or extraction according to the determined ion concentration.

FIG. 1A illustrates a system 10A for measuring an ion concentration of asample fluid 150 using an optical computing device 101A including an ICE102. Optical computing device 101A includes an ion-selective membrane100 arranged within an optical path of optical computing device 101A,and coupled to a sample cell 155. Sample cell 155 allows sample fluid150 to interact with ion-selective membrane 100 to modify an opticalresponse of ion-selective membrane 100 according to an ion concentrationin sample fluid 150. In that regard, ion-selective membrane 100 may bean ion sensitive ion-selective membrane that absorbs a pre-selected typeof ion from sample fluid 150 when ion-selective membrane 100 makescontact with the sample fluid 150. Accordingly, the affinity ofion-selective membrane 100 for the pre-selected ion depends on thematerial forming ion-selective membrane 100; including its structuralcharacteristics, such as porosity, the ion charge, the ion mass, thesolvent carrying the ion in the fluid (e.g., water, alcohol), and thepresence of other ions in the solution (e.g., the pH of a watersolution). The affinity of ion-selective membrane 100 for ions in samplefluid 150 also depends on various environmental factors such astemperature and pressure of sample fluid 150.

The optical path in optical computing device 101A is defined by anillumination light 141 generated by an optical source 140. Sample cell155 helps facilitate optical interaction of illumination light 141 withion-selective membrane 100, thus generating sample light 142. Morespecifically, sample cell 155 provides a location for the opticalinteraction between illumination light 141 and ion-selective membrane100 to take place. In some embodiments, optical source 140 may be abroadband lamp, a laser, a light-emitting diode, or any other source ofelectromagnetic radiation. In some embodiments, sample light 142 mayinclude fluorescence emitted photons or Raman shifted photons fromsample fluid 150.

Integrated computational element (ICE) 102 optically interacts withsample light 142 to provide modified light 143. A property of modifiedlight 143 is indicative of a characteristic of sample fluid 150, such asthe ion concentration in sample fluid 150. In some embodiments, theproperty of the modified light that is indicative of the ionconcentration in sample fluid 150 may be an intensity, a polarizationstate, a phase, a wavelength, or any combination of the above. Opticalcomputing device 101A also includes a detector 130 that receivesmodified light 143 and provides an electrical signal to a controller160. In some embodiments, the electrical signal is proportional to theproperty of modified light 143.

In some embodiments, ICE 102 is configured so that the irradiance ofmodified light 143 is proportional to the absorbed value of the ion inion-selective membrane 100. Accordingly, the absorbed value of the ionin ion-selective membrane 100 may be determined from the amount ofmodified light 143 arriving to detector 130. In addition, the ionconcentration in sample fluid 150 is associated with the ion adsorptionin ion-selective membrane 100 from a known correlation between the twovalues. More generally, the position of ICE 102 relative to sample cell155 may be interchangeable. Accordingly, in some embodiments ICE 102 maybe disposed between optical source 140 and sample cell 155, generatingmodified light 143 directly from illumination light 141. In suchembodiments, ion-selective membrane 100 interacts with modified light143 to generate sample light 142, which is then measured by detector130.

According to embodiments consistent with FIG. 1A, at least one side ofsample cell 155 is coupled with ion-selective membrane 100. For example,in some embodiment a window in sample cell 155 may be coated on theinside (the side that comes into contact with sample fluid 150) with alayer of material forming ion-selective membrane 100. The opticalabsorption spectrum of ion-selective membrane 100 changes in response tothe pre-selected ion. As fluid 150 moves through sample cell 155, itinteracts with ion-selective membrane 100, changing the absorptionspectrum of sample cell 155 in response to the ion concentration influid 150. Accordingly, the intensity of light hitting detector 130 isindicative of the pre-selected ion concentration in sample fluid 150. Inthat regard, sample fluid 150 may be transparent to illumination light141. For example, in some embodiments illumination light 141 onlyinteracts with ion-selective membrane 100.

In some embodiments, sample cell 155 may include a relatively thin fluidpassage (about 1 mm thick), through which at least a portion of samplefluid 150 passes for measurement. This may include a wireline-deployedtool such as a reservoir description tool (RDT™), where a pump is usedto force fluid 150 through sample cell 155. A controller 160 has aprocessor 161 and a memory 162. Memory 162 stores data and commandswhich, when executed by processor 161, cause controller 160 to directsystem 10A to perform steps in methods consistent with the presentdisclosure.

FIG. 1B shows a system 10B for measuring an ion concentration of asample fluid 150 using an optical computing device 101B. Elements inFIG. 1B having the same reference numeral as in FIG. 1A have the samedetailed description as given above, and their description will not berepeated hereinafter. In optical computing device 101B, detector 130 isconfigured to measure sample light 142 directly and provide an electricsignal to controller 160 for processing. In that regard, sample light142 may provide direct information of the absorbed value of the ion inion-selective membrane 100, for example when sample light 142 is afluorescent emission light, or a Raman shifted light. In suchconfigurations, the intensity of the emitted fluorescent light or theintensity of the Raman light may be proportional to the ionconcentration in ion-selective membrane 100. Moreover, a wavelengthshift in the fluorescence emission or in the Raman emission may beindicative of the ion adsorption value in ion-selective membrane 100. Insome embodiments, an optical filter (not shown) is used in front ofdetector 130 to select the range of wavelengths of the sample lightneeded for the measurement (e.g. Raman spectrum) while removing otherwavelengths (e.g., original excitation laser wavelength).

FIG. 2 illustrates a cross-sectional view of an exemplary integratedcomputational element (ICE) 202 for measuring an ion concentration insample fluid 150. ICE 202 may be similar to or the same as ICE 102 ofFIG. 1A and, therefore, may be used in optical computing device 101A ofFIG. 1A. As illustrated, ICE 202 may include a plurality of alternatinglayers 203 and 204, such as silicon (Si) and SiO₂ (quartz),respectively. In general, layers 203, and 204 include materials whoseindex of refraction is high and low, respectively. Other examples ofmaterials for use in layers 203 and 204 might include niobia andniobium, germanium and germania, MgF, SiO, and other high and low indexmaterials known in the art. Layers 203, 204 may be strategicallydeposited on an optical substrate 206. In some embodiments, the opticalsubstrate 206 is BK-7 optical glass. In other embodiments, opticalsubstrate 206 may be another type of optical substrate, such as quartz,sapphire, silicon, germanium, zinc selenide, zinc sulfide, or variousplastics such as polycarbonate, polymethylmethacrylate (PMMA),polyvinylchloride (PVC), diamond, ceramics, combinations thereof, andthe like.

At the opposite end (e.g., opposite optical substrate 206 in FIG. 2 ),ICE 202 may include a layer 208 that is generally exposed to theenvironment of the device or installation, and may be able to detect asample substance. In some embodiments, layer 208 may include anion-selective membrane (e.g., ion-selective membranes 100, 100 a, 100 bcf. FIGS. 1A-1B). In such embodiments, ICE 202 may be disposed in samplecell 155 such that layer 208 makes contact with or is in close proximityto sample fluid 150, thereby allowing the pre-selected ions to beabsorbed in layer 208. In some embodiments, ICE 202 may be deposited onan outer surface of a window in sample cell 155 (cf. FIGS. 1A-1B), andion-selective membrane 100 may be deposited on an inner surface of thesame window. In such embodiments, the window in sample cell 155 may actas a rigid substrate from both ICE 202 and ion-selective membrane 100.

The number of layers 203, 204 and the thickness of each layer 203, 204are determined from the spectral attributes acquired from aspectroscopic analysis of a characteristic of the substance beinganalyzed using a conventional spectroscopic instrument. The spectrum ofinterest of a given characteristic typically includes any number ofdifferent wavelengths. It should be understood that ICE 202 in FIG. 2does not in fact represent any particular characteristic of a givensubstance, but is provided for purposes of illustration only.Consequently, the number of layers 203, 204 and their relativethicknesses, as shown in FIG. 2 , bear no correlation to any particularcharacteristic. Nor are layers 203, 204 and their relative thicknessesnecessarily drawn to scale, and therefore should not be consideredlimiting of the present disclosure. Moreover, those skilled in the artwill readily recognize that the materials that make up each layer 203,204 (i.e., Si and SiO₂) may vary, depending on the application, cost ofmaterials, and/or applicability of the material to the given substancebeing analyzed.

In some embodiments, the material of each layer 203, 204 can be doped ortwo or more materials can be combined in a manner to achieve the desiredoptical characteristic. In addition to solids, ICE 202 may also containliquids and/or gases, optionally in combination with solids, in order toproduce a desired optical characteristic. In the case of gases andliquids, ICE 202 can contain a corresponding vessel (not shown), whichhouses the gases or liquids. Exemplary variations of ICE 202 may alsoinclude holographic optical elements, gratings, frequency selectivesurfaces, piezoelectric, light pipe, and/or acousto-optic elements, forexample, that can create transmission, reflection, and/or absorptiveproperties of interest.

Layers 203 and 204 exhibit different refractive indices. By properlyselecting the materials of layers 203, 204 and their relative thicknessand spacing, ICE 202 may be configured to selectivelypass/reflect/refract predetermined fractions of electromagneticradiation at different wavelengths. Each wavelength is given apredetermined weighting or loading factor. The thickness and spacing oflayers 203, 204 may be determined using a variety of approximationmethods from the spectrum of the characteristic or analyte of interest.These methods may include inverse Fourier transform (IFT) of the opticaltransmission spectrum and structuring ICE 100 as the physicalrepresentation of the IFT. The approximations convert the IFT into astructure based on known materials with constant refractive indices.

The weightings that layers 203, 204 of ICE 202 apply at each wavelengthare set to the regression weightings described with respect to a knownequation, or data, or spectral signature. When electromagnetic radiationinteracts with a substance, unique physical and chemical informationabout the substance may be encoded in the electromagnetic radiation thatis reflected from, transmitted through, or radiated from the substance.This information is often referred to as the spectral “fingerprint” ofthe substance. ICE 202 performs the dot product of the electromagneticradiation received by ICE 202 (e.g., sample light 142, cf. FIG. 1A) andthe wavelength dependent transmission function of ICE 202. Thewavelength dependent transmission function of ICE 202 is dependent onthe layer material refractive index, the number of layers 203, 204 andthe layer thicknesses. The transmission function of ICE 202 is designedto mimic a desired regression vector derived from the solution to alinear multivariate problem targeting a specific component of the samplebeing analyzed. As a result, the output light intensity of ICE 202(e.g., the intensity of modified light 143, cf. FIG. 1A) is proportionala dot product of a transmission spectrum of the sample with theregression vector associated with the characteristic of interest.Accordingly, the output light intensity of ICE 202 is a direct indicatorof a value of the characteristic of interest of the sample.

Referring again to FIG. 1A, optical computing device 101A employing ICE102 may be capable of extracting the information of the spectralfingerprint of multiple characteristics or analytes within a substance(e.g., ions dissolved in sample fluid 150) and converting thatinformation into a detectable output regarding the overall properties ofthe substance. That is, through suitable configurations of the opticalcomputing device 101A, electromagnetic radiation associated withcharacteristics or analytes of interest in a substance (e.g., ionsdissolved in the sample fluid 150) can be separated from electromagneticradiation associated with all other components of the substance in orderto estimate the properties of the substance in real-time or nearreal-time. Accordingly, ICE 102 is able to distinguish and processelectromagnetic radiation related to a characteristic or analyte ofinterest.

In that regard, optical source 140 may be configured such that samplefluid 150 is transparent to illumination light 141. Accordingly, samplelight 142 differs from illumination light 141 only by the opticalinteraction with ion-selective membrane 100.

FIG. 3 illustrates a chart 300 with absorbance spectra 301, 302, and 303of sample light 142 interacted with ion-selective membrane 100 when thesample (e.g., the sample fluid 150 of FIG. 1A) includes three differention concentrations. The abscissae in chart 300 correspond to wavelength(in arbitrary units), and the ordinates in chart 300 correspond toabsorbance (in arbitrary units). Without limitation, the ordinate axisin chart 300 may indicate a fluorescence or a Raman emission amplitude,and the abscissae may indicate a Raman shift instead of a wavelength.

Spectra 301, 302 and 303 each cover a wavelength range from a minimumwavelength λ_(min), to a maximum wavelength λ_(max). In someembodiments, the wavelength span between λ_(min), and λ_(max) includethe visible wavelength range (from about 400 nm to about 750 nm). Moregenerally, either one of λ_(min) or λ_(max) may be outside the visiblewavelength range, such as the ultra-violet UV wavelength range (fromabout 290 nm to about 400 nm) and the near-infrared wavelength range(from about 750 nm to about 2500 nm).

Spectrum 301 represents zero or very low concentrations of thepre-selected ion, while a high concentration of the pre-selected ionresults in spectrum 302. Spectrum 303 corresponds to an intermediate ionconcentration between the high (302) and low (301) ion concentrations influid 150. The ions being detected in this case are positively-chargedand their presence in the fluid results in an exchange with a hydrogenion (also positively charged) bonded to a chromo-ionophore (dye)molecule in ion-selective membrane 100. Spectra 301 and 302 are markedlydifferent from one another, and therefore suitable for the spectralregression analysis with ICE 102 to determine the absorbed value of theion in ion-selective membrane 100.

An advantage of system 10A is the capability of ion-selective membrane100 to operate at high temperatures such as 100° C., 150° C., 200° C.,250° C. or even higher. The high temperature resilience of ion-selectivemembrane 100 is a desirable property for downhole measurement tools. Thespeed at which ion-selective membrane 100 reaches equilibrium with ionsdissolved in fluid 150, and the amount of ions absorbed intoion-selective membrane 100 upon equilibrium is determined by theequilibrium constant K_(a) of ion-selective membrane 100. The K_(a)constant is a factor that generally depends on the temperature, pressureand pH of sample fluid 150, and also on other conditions such as thematerial forming ion-selective membrane 100, its porosity, mass, andcharge of the pre-selected ion, among other factors. In that regard,some embodiments of ion-selective membranes 100, 100 a, and 100 b mayinclude a diffusion barrier to delay the absorption of the pre-selectedions from sample fluid 150. In such embodiments, the optical measurementdescribed in system 10A or in system 40A may be performed before the ionconcentration between sample fluid 150 and ion-sensitive membrane 100has reached equilibrium. Accordingly, controller 160 may be configuredto measure the speed, or rate of ion absorption into ion-sensitivemembrane 100 from sample fluid 150. In turn the speed or rate ofabsorption may be used to determine a characteristic of sample fluid150, such as a water concentration (i.e. “water cut” measurement).

FIG. 4 illustrates a system 40 for measuring a plurality of ionconcentrations of sample fluid 150 using an optical computing device401. Elements in system 40 having the same reference numeral as elementsin system 10A have the same configuration and description as providedabove.

System 40 includes two ion-sensitive ion-selective membranes 100 a and100 b, each with an affinity optimized for a different ion. For example,ion-selective membrane 100 a may be optimized for Ca⁺² (calcium) whereasion-selective membrane 100 b may be optimized for K⁺ (potassium).Beamsplitter 410 separates illumination light 141 into a first portionreflected by mirror 411 and passing through ion-selective membrane 100a, and a second portion passing through ion-selective membrane 100 b. Inthat regard, sample light 142 a may include a response fromion-selective membrane 100 a and sample light 142 b may include aresponse from ion-selective membrane 100 b. Each ICE 102 a and 102 boptically interacts with sample light 142 a and 142 b, providingmodified light 143 a and 143 b to detectors 130 a and 130 b,respectively. Modified light 143 a includes a property indicative of aCa⁺² concentration and modified light 143 b includes a propertyindicative of a K⁺ concentration in fluid 150. In some embodiments, ICEs102 a and 102 b may be replaced by standard bandpass filters, or may betaken out from optical computing device 401 altogether.

In some embodiments, for example, ion-selective membrane 100 a may havea neutral response to sample fluid 150 and ICE 102 a may be replaced bya neutral density filter. Accordingly, modified light 143 a may be areference light, and detector 130 a may provide a reference measurementto controller 160.

The above-described embodiments may be applied to any two ionconcentrations other than Ca⁺² and K⁺. Further, the number of differentions measured is not limiting of the embodiments disclosed herein andtwo, three, or more different ions may be pre-selected for measurement.

FIG. 5A illustrates a system 50 for measuring an ion concentration ofsample fluid 150 using an optical computing device 501 including anoptical waveguide 510. Ion-selective membrane 100 forms a layerinterposed in the propagation path of optical waveguide 510, forming asample cell 555. In some embodiments, sample cell 555 may be dipped intosample fluid 150, rapidly generating an ion-exchange with ion-selectivemembrane 100, in at least a sample portion of the waveguide.Accordingly, ion-selective membrane 100 may be arranged within thesample portion of the waveguide to facilitate the ion exchange.Accordingly, in a tool with a large flow area such as in a permanentcompletion wellbore, sample cell 555 is easily exposed to sample fluid150, rather than coaxing a properly mixed, representative sample to flowthrough a tiny sample cell 155. Waveguide 510 may be an optical fiberhaving a diameter of a few tens of microns (1 μm=10⁻⁶ m), a glass rodhaving a thicker diameter than a fiber, a light pipe of a few mm indiameter, or any type of optical waveguide configured to propagateillumination light 141 with a low loss.

In some embodiments of system 50, waveguide 510 includes a length ofsmall diameter fiber or other optical waveguide wherein a short,longitudinal section of ion-selective membrane 100 is sandwiched between(interposes) two axially adjacent portions of the waveguide 510.Focusing optics 511 convey illumination light 141 into waveguide 510 andexpanding optics 512 receive and expand the resulting sample light 142,and provide sample light 142 to ICE 102 before reaching detector 130. Insome embodiments, optical source 140 and optical computing device 501are contained within a pressure vessel. Thus, the flexibility ofwaveguide 510 enables adapting optical computing device 501 to anyarbitrary shape with waveguide 510 routed outside of the pressure vesseland exposed to a large diameter flow area inside the wellbore. Wellborefluids (e.g., sample fluid 150) can wash over waveguide 510 and therebyinteract with ion-selective membrane 100. Ions in fluid sample 150change the optical absorption spectrum of ion-selective membrane 100 andallow ICE 102 and detector 130 to determine the concentration of thetarget ion (cf. spectra 301, 302, and 303, cf. FIG. 3 ).

FIG. 5B illustrates optical waveguide 510 including ion-selectivemembrane 100 for use in optical computing device 501 (FIG. 5A) formeasuring an ion concentration of sample fluid 150. FIG. 5B shows a moredetailed schematic of sample cell 555, which, in some embodiments,includes a protective sleeve 520 covering waveguide 510 and providingone or more slots 522 to allow sample fluid 150 to interact withion-selective membrane 100. Since the diameter of waveguide 510 issmall, ion-selective membrane 100 absorbs ions from sample fluid 150,altering its optical absorbance/transmittance spectrum. Some embodimentsinclude additional support on either end of waveguide 510 to keepoptical computing device 501 together, even without adhesively bondingion-selective membrane 100 to the two waveguide portions shown in thefigure. For example, ion-selective membrane 100 may be painted, coated,or deposited on one side of waveguide 510, while the other side ofwaveguide 510 is abutted on the free side of ion-selective membrane 100.

The material forming ion-selective membrane 100 may have in general adifferent index of refraction (n_(e)) than that of waveguide 510. Thismay produce fringing effects in sample light 142, as in a Fabry-Perotinterferometer. In some embodiments, illumination light 141 is abroadband light with a coherence length desirably shorter than thethickness of ion-selective membrane 100. Some embodiments may tolerate alimited degree of interference fringes resulting from the finitethickness of ion-selective membrane 100. To mitigate interference andloss effects, in some embodiments it is desirable that the thickness ofion-selective membrane 100 in waveguide 510 be larger than the coherencelength of illumination light 141, or the propagating wavelength ofillumination light 141. In other embodiments, the surfaces ofion-selective membrane 100 are not parallel in order to avoidFabry-Pérot effects.

FIG. 6 illustrates a system 60 for measuring an ion concentration ofsample fluid 150 using optical computing device 601. Optical computingdevice 601 includes an ion-selective membrane 600 that interacts withsample fluid 150 in a first region 610 and interacts with illuminationlight 141 in a second region 611. In system 60, ion-selective membrane600 is disposed along the optical path (train) of illumination light141. ICE 102 interacts with sample light 142 adjacent region 611 toproduce modified light 143, which is subsequently detected by detector130. Ion-selective membrane 600 extends beyond the optical path ofillumination light 141 into region 610, where it is able to physicallyinteract with fluid 150. Ion-selective membrane 600 absorbs ions fromfluid 150, which diffuse from region 610 into region 611. Sample light142 is generated when illumination light 141 interacts withion-selective membrane 600 in region 611.

Detector 130 may be configured to perform measurements after equilibriumin between ions absorbed in ion-selective membrane 100 and the ionconcentration in sample fluid 150 has been reached. Alternatively,detector 130 may perform measurements during an ion diffusion transientbetween ion-selective membrane 100 and fluid 150. For example, in someembodiments detector 130 may describe in detail the transient speed,which may be indicative of a characteristic of fluid 150. For example,in some embodiments the transient speed may be associated with thepresence of water or oil in fluid 150. In that regard, a highertransient speed fluid 150 may be associated with a fluid 150 having morewater, or less oil, than a slower transient speed fluid 150.

FIG. 7 illustrates a wireline system 700 configured to measure an ionconcentration of a sample fluid during formation testing and samplingwith an optical computing device. After drilling of wellbore 718 iscomplete, it may be desirable to know more details of types of formationfluids and the associated characteristics through sampling with use of awireline formation tester. System 700 may include a wireline loggingtool 712 that forms part of a wireline logging operation that caninclude one or more optical computing devices 714 as described herein(e.g., optical computing device 101, 401, 501 or 601 cf. FIGS. 1, 4, 5and 5 ). Accordingly, any one of optical computing devices 714 mayinclude an ICE according to embodiments disclosed herein (e.g., ICE 202,cf. FIG. 2 ), and an ion-selective membrane (e.g., ion-selectivemembranes 100 and 600, cf. FIGS. 1, 4 and 5 ). System 700 may include aderrick 702 that supports a traveling block 704. Wireline logging tool712, such as a probe or sonde, may be lowered by wireline or loggingcable 706 into borehole 718. Tool 712 may be lowered to the bottom ofthe region of interest and subsequently pulled upward at a substantiallyconstant speed by wireline or logging cable 606.

Any measurement data generated by wireline logging tool 712 and itsassociated optical computing devices 714 can be communicated to asurface logging facility 708 for storage, processing, and/or analysis.Logging facility 708 may be provided with controller 760, includingprocessor 761 and memory 762 configured to perform various types ofsignal processing. Controller 760 may determine a reservoir qualitybased on a measurement of the ion concentration of an extracted samplefluid from wellbore 718. The reservoir quality may be associated withthe ability to extract a high value hydrocarbon from a formationsurrounding the wellbore. Many factors may determine a reservoirquality, including pore geometry in the formation surrounding thewellbore and other physical properties of the fluid such as viscosity,density, pH, and ion concentration, among others. Controller 760 maydetermine at least some of the above physical properties usingmeasurements obtained with optical computing devices 714 as disclosedherein.

FIG. 8 illustrates a field deployment of a fluid analysis system 800including multiple optical computing devices 801 deployed in wellbore818 for measuring an ion concentration of a sample fluid. Wellbore 818may include a plurality of extraction reservoirs including hydrocarbonproduction zones 850 a, 850 b, 850 c, and 850 d, as illustrativeexamples (collectively referred to hereinafter as production zones 850).For example, in some embodiments the ion concentration in fluidsextracted from each of production zones 850 may be different, andindicative of each specific location. In that regard, some embodimentsof fluid analysis system 800 include specifically injecting a fluidhaving a recognizable ion concentration into a distant well (not shown)which is detected in wellbore 818. Fluids containing different ions maybe injected from different wells around the wellbore of interest, 818.Accordingly, measuring the ion concentration of an extracted fluid mayindicate the origin of the fluid (i.e., whether or not is the same fluidas was originally injected into the wellbore). Moreover, the ionconcentration in the extracted fluid may indicate whether the fluid wasextracted from either one of production zones 850 a, 850 b, 850 c, or850 d. In some embodiments, fluid analysis system 800 is deployed forlong periods of time such as months, years, or even longer periods oftime as allowed by the resiliency of optical computing devices 801 overharsh environmental conditions.

At least one of optical computing devices 801 includes an ion-selectivemembrane as disclosed herein (cf. ion-selective membranes 100 and 600,cf. FIGS. 1 and 6 ). In fluid analysis system 800, a derrick 802provides support for hydrocarbon extraction and measurement equipmentdeployed through wellbore 818. At the surface, fluid analysis system 800may include a controller 860 having a processor 861 and a memory 862.Controller 860, processor 861, and memory 862 may be as described indetail above (e.g., controllers 160 and 760, processors 161 and 761, andmemories 162 and 762 respectively, cf. FIGS. 1 and 7 ). In someembodiments, controller 860 is configured to determine the origin of anextracted fluid from the ion concentration measured with each of opticalcomputing devices 801. Accordingly, controller 860 may be configured todetermine whether an extracted fluid comes from production zone 850 a,850 b, 850 c, or 850 d, or if it was injected at the surface.

Wellbore 818 may be a subterranean wellbore or an undersea operation, inwhich case derrick 802, controller 860, and a portion of a pipeline 803may be floating over the sea. The extracted hydrocarbon is transportedthrough pipeline 803 to a delivery port 810, from which the hydrocarbonis transferred to a transportation vehicle (e.g., vessel 833 or truck835), a refinery 837, or a power plant 839, among others.

FIG. 9 illustrates a flow chart including steps in a method 900 formeasuring an ion concentration in a sample fluid. In some embodiments,steps in method 900 may be performed at least partially by a controllerincluding a processor and a memory (e.g., controllers 160, 760, or 860,processors 161, 761, or 861, and memories 162, 762, and 862, cf. FIGS.1A-B, 4, 7, and 8). The memory may store commands that, when executed bythe processor, cause the controller to perform at least some of thesteps in method 900. Accordingly, methods consistent with method 900 maybe performed in connection with a system including an optical computingdevice having an ICE and an ion-selective membrane (e.g., systems 10,40, 50 and 60, ICE 102 and ion-selective membranes 100 and 600, cf.FIGS. 1A-B, 4, 5A, and 6). Moreover, methods consistent with method 900may include using a optical source to provide an illumination light forthe optical computing device, an optical waveguide such as an opticalfiber, and a detector (e.g., optical source 140, illumination light 141,optical waveguide 510 and detector 130, cf. FIGS. 1 and 5 ).

Methods consistent with method 900 may include fewer steps thanillustrated in FIG. 9 , or other steps in addition to at least one ofthe steps in method 900. Moreover, methods consistent with the presentdisclosure may include at least one or more of the steps in method 900performed in a different sequence. For example, some embodimentsconsistent with the present disclosure may include at least two steps inmethod 900 performed overlapping in time, or substantiallysimultaneously in time.

Step 902 includes exchanging ions between a material and a sample. Insome embodiments, step 902 may include exchanging ions between thematerial and the sample to reach equilibrium. In other embodiments, step902 is taken before the sample has reached equilibrium with itssurroundings so that a transient response is obtained (e.g., in watercut measurements). In some embodiments, step 902 includes exchangingions between a second material and the sample, wherein the secondmaterial has an affinity optimized for a second ion. Step 904 includesoptically interacting the material with an ICE to generate a modifiedlight. Step 906 includes detecting a change in the optical response ofthe material based on the modified light. In some embodiments, step 806includes measuring a property of the modified light. For example, insome embodiments step 906 includes measuring at least one of anintensity, a polarization, a phase, a wavelength or any combination ofthe above properties of the modified light.

Step 908 includes determining an ion concentration in the sample fromthe change in the optical response of the material. Step 908 may includedetermining first an absorbed value of the ion in the material from thechange in the optical response of the material. Further, step 908 mayinclude determining the ion concentration in the sample from theabsorbed value assuming ion-transmission equilibrium between thematerial and the sample. In some embodiments, step 908 includesidentifying a temperature, a pressure, and a pH of the sample as factorsin determining the ion concentration in the sample. In some embodiments,step 908 may include determining the origin of an extracted fluid fromthe ion concentration in the sample. Accordingly, the origin of theextracted fluid may be any one of a plurality of production zones in awellbore (e.g., production zones 850, cf. FIG. 8 ), or a fluid injectedinto the wellbore from the surface. Step 910 includes modifying anoperational parameter in a wellbore for oil and gas exploration orextraction according to the determined ion concentration. In someembodiments, step 910 may include adding into or removing from aninjection fluid certain chemical compounds, in order to modify the pH ofthe injection fluid, or its reactivity with an membrane in the wellbore.For example, the injection fluid may be a drilling mud and its ioncontent may indicate the chemical stability of a wellbore ion-selectivemembrane while drilling.

Those skilled in the art will readily appreciate that the methodsdescribed herein, or large portions thereof may be automated at somepoint such that a computerized system may be programmed to transmit datafrom an optical computing device using an ICE element. Computer hardwareused to implement the various methods and algorithms described hereincan include a processor configured to execute one or more sequences ofinstructions, programming stances, or code stored on a non-transitory,computer-readable medium. The processor can be, for example, a generalpurpose microprocessor, a microcontroller, a digital signal processor,an application specific integrated circuit, a field programmable gatearray, a programmable logic device, a controller, a state machine, agated logic, discrete hardware components, an artificial neural network,or any like suitable entity that can perform calculations or othermanipulations of data. In some embodiments, computer hardware canfurther include elements such as, for example, a memory (e.g., randomaccess memory (RAM), flash memory, read only memory (ROM), programmableread only memory (PROM), electrically erasable programmable read onlymemory (EEPROM)), registers, hard disks, removable disks, CD-ROMS, DVDs,or any other like suitable storage device or medium.

Executable sequences described herein can be implemented with one ormore sequences of code contained in a memory. In some embodiments, suchcode can be read into the memory from another machine-readable medium.Execution of the sequences of instructions contained in the memory cancause a processor to perform the process steps described herein. One ormore processors in a multi-processing arrangement can also be employedto execute instruction sequences in the memory. In addition, hard-wiredcircuitry can be used in place of or in combination with softwareinstructions to implement various embodiments described herein. Thus,the present embodiments are not limited to any specific combination ofhardware and/or software.

As used herein, a machine-readable medium will refer to any medium thatdirectly or indirectly provides instructions to a processor forexecution. A machine-readable medium can take on many forms including,for example, non-volatile media, volatile media, and transmission media.Non-volatile media can include, for example, optical and magnetic disks.Volatile media can include, for example, dynamic memory. Transmissionmedia can include, for example, coaxial cables, wire, fiber optics, andwires that form a bus. Common forms of machine-readable media caninclude, for example, floppy disks, flexible disks, hard disks, magnetictapes, other like magnetic media, CD-ROMs, DVDs, other like opticalmedia, punch cards, paper tapes and like physical media with patternedholes, RAM, ROM, PROM, EPROM and flash EPROM.

Therefore, the present disclosure is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent disclosure may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered,combined, or modified and all such variations are considered within thescope and spirit of the present disclosure. The disclosureillustratively disclosed herein suitably may be practiced in the absenceof any element that is not specifically disclosed herein and/or anyoptional element disclosed herein. While compositions and methods aredescribed in terms of “comprising,” “containing,” or “including” variouscomponents or steps, the compositions and methods can also “consistessentially of” or “consist of” the various components and steps. Allnumbers and ranges disclosed above may vary by some amount. Whenever anumerical range with a lower limit and an upper limit is disclosed, anynumber and any included range falling within the range is specificallydisclosed. In particular, every range of values (of the form, “fromabout a to about b,” or, equivalently, “from approximately a to b,” or,equivalently, “from approximately a-b”) disclosed herein is to beunderstood to set forth every number and range encompassed within thebroader range of values. Also, the terms in the claims have their plain,ordinary meaning unless otherwise explicitly and clearly defined by thepatentee. Moreover, the indefinite articles “a” or “an,” as used in theclaims, are defined herein to mean one or more than one of the elementthat it introduces. If there is any conflict in the usages of a word orterm in this specification and one or more patent or other documentsthat may be incorporated herein by reference, the definitions that areconsistent with this specification should be adopted.

As used herein, the phrase “at least one of” preceding a series ofitems, with the terms “and” or “or” to separate any of the items,modifies the list as a whole, rather than each member of the list (i.e.,each item). The phrase “at least one of” allows a meaning that includesat least one of any one of the items, and/or at least one of anycombination of the items, and/or at least one of each of the items. Byway of example, the phrases “at least one of I, II, and III” or “atleast one of I, II, or III” each refer to only I, only II, or only III;any combination of I, II, and III; and/or at least one of each of I, II,and III.

Embodiments disclosed herein include:

A. A device including an ion-selective membrane arranged within anoptical path of the device and coupled to a sample cell to interact witha fluid sample and thereby modify an optical response of theion-selective membrane according to an ion concentration in the fluidsample. The optical path of the device is defined by an illuminationlight. The device includes an integrated computational element (ICE)arranged within the optical path, wherein the illumination lightoptically interacts with the ICE and with the ion-selective membrane toprovide a modified light that has a property indicative of the ionconcentration in the fluid sample. In some embodiments, the deviceincludes a detector that receives the modified light and provides anelectrical signal proportional to the property of the modified light.

B. A device including an optical waveguide that transmits anillumination light to a detector, and that couples with a fluid samplein at least one sample portion of the waveguide. The device includes anion-selective membrane arranged within the sample portion of thewaveguide to interact with the fluid sample and thereby modify anoptical response of the ion-selective membrane according to an ionconcentration in the fluid sample. In some embodiments, the deviceincludes an integrated computational element (ICE) arranged within anoptical path defined by the illumination light to provide a modifiedlight that has a property indicative of the ion concentration in thefluid sample. In some embodiments, the detector receives the modifiedlight and provides an electrical signal proportional to the property ofthe modified light.

C. A method including exchanging ions between a material and a fluidsample, optically interacting the material with an illumination lightgenerated by an optical source to generate a modified light anddetecting a change in the optical response of the material based on themodified light. In some embodiments, the method includes determining anion concentration in the fluid sample from the change in the opticalresponse of the material and modifying an operational parameter in awellbore for oil and gas exploration or extraction according to thedetermined ion concentration.

Each of embodiments A, B and C may have one or more of the followingadditional elements in any combination: Element 1, wherein the samplecell facilitates optical interaction between the illumination light andthe ion-selective membrane to generate a sample light, and the samplelight interacts with the ICE to provide the modified light. Element 2,wherein the ion-selective membrane absorbs a pre-selected type of ionsfrom the fluid sample up to an equilibrium value determined by the ionconcentration in the sample. Element 3, including a second ion-selectivemembrane that absorbs a pre-selected, second type of ions from the fluidsample. Element 4, wherein the sample light is at least one of afluorescent emission and a Raman emission. Element 5, wherein theproperty of the modified light is selected from a group consisting of anintensity, a polarization state, a phase, and a wavelength. Element 6,wherein the ion-selective membrane has an affinity optimized for an ionselected from the group consisting of Na⁺, Ca⁺² K⁺, Mg⁺², Cr⁺³, HC0₃ ⁻,SO⁻, NH⁺, and N0₃ ⁻. Element 7, wherein the ion-selective membranecontacts the fluid sample at a first region and the ion-selectivemembrane interacts with the illumination light at a second regionseparated from the first region. Element 8, including at least a secondion-selective membrane that absorbs a second ion from the fluid sampleand interacts with the illumination light. Element 9, wherein the deviceis an optical computing device in a system, the system furtherincluding: an optical source that provides the illumination light, and acontroller communicable coupled to the optical computing device, whereinthe controller determines the ion concentration in the fluid sample fromthe property of the modified light. Element 10, wherein the opticalsource includes one selected from the group consisting of a broadbandlamp, a laser, and a light-emitting diode. Element 11, wherein thecontroller determines the ion concentration based on at least one of atemperature, a pressure, or a pH of the fluid sample.

Element 12, wherein the ion-selective membrane forms a layer interposedin a propagation path of the illumination light along the waveguide, anda thickness of the layer is larger than a coherence length of theillumination light.

Element 13, wherein detecting a change in the optical response of thematerial includes measuring a property of the modified light selectedfrom the group consisting of an amplitude, a polarization state, aphase, a wavelength or a combination thereof. Element 14, whereinexchanging ions between the material and the fluid sample includesallowing for the ion exchange between the material and the fluid sampleto reach equilibrium. Element 15, wherein determining the ionconcentration in the fluid sample includes identifying a temperature, apressure, and a pH of the fluid sample. Element 16, further includingexchanging ions between a second material and the fluid sample, whereinthe second material has an affinity optimized for a second ion.

The invention claimed is:
 1. A device, comprising: an illumination lightdefining an optical path of the device; an ion-selective membranearranged within the optical path of the device and coupled to a samplecell to interact with a fluid sample and thereby modify an opticalresponse of the ion-selective membrane according to an ion concentrationin the fluid sample; an integrated computational element (ICE) arrangedwithin the optical path, wherein the illumination light opticallyinteracts with the ICE and with the ion-selective membrane to provide amodified light that has a property indicative of the ion concentrationin the fluid sample; and a detector that receives the modified light andprovides an electrical signal proportional to the property of themodified light.
 2. The device of claim 1 wherein the sample cellfacilitates optical interaction between the illumination light and theion-selective membrane to generate a sample light, and the sample lightinteracts with the ICE to provide the modified light.
 3. The device ofclaim 1, wherein the ion-selective membrane absorbs a pre-selected typeof ions from the fluid sample up to an equilibrium value determined bythe ion concentration in the sample.
 4. The device of claim 1, furtherincluding a second ion-selective membrane that absorbs a pre-selected,second type of ions from the fluid sample.
 5. The device of claim 1,wherein the sample light is at least one of a fluorescent emission and aRaman emission.
 6. The device of claim 1, wherein the property of themodified light is selected from a group consisting of an intensity, apolarization state, a phase, and a wavelength.
 7. The device of claim 1,wherein the ion-selective membrane has an affinity optimized for an ionselected from the group consisting of Na⁺, Ca⁺² K⁺, Mg⁺², Cr⁺³, HC0₃ ⁻,SO⁻, NH⁺, and N0₃ ⁻.
 8. The device of claim 1, wherein the ion-selectivemembrane contacts the fluid sample at a first region and theion-selective membrane interacts with the illumination light at a secondregion separated from the first region.
 9. The device of claim 1,further comprising at least a second ion-selective membrane that absorbsa second ion from the fluid sample and interacts with the illuminationlight.
 10. The device of claim 1, wherein the device is an opticalcomputing device in a system, the system further comprising: an opticalsource that provides the illumination light; and a controllercommunicable coupled to the optical computing device, wherein thecontroller determines the ion concentration in the fluid sample from theproperty of the modified light.
 11. The device of claim 10, wherein theoptical source comprises one selected from the group consisting of abroadband lamp, a laser, and a light-emitting diode.
 12. The device ofclaim 10, wherein the controller determines the ion concentration basedon at least one of a temperature, a pressure, or a pH of the fluidsample.
 13. A device, comprising: an illumination light defining anoptical path of the device; an optical waveguide that transmits theillumination light to a detector, and that couples with a fluid samplein at least one sample portion of the waveguide; an ion-selectivemembrane arranged within the sample portion of the waveguide to interactwith the fluid sample and thereby modify an optical response of theion-selective membrane according to an ion concentration in the fluidsample; and an integrated computational element (ICE) arranged withinthe optical path defined by the illumination light to provide a modifiedlight that has a property indicative of the ion concentration in thefluid sample; wherein the detector receives the modified light andprovides an electrical signal proportional to the property of themodified light.
 14. The device of claim 13, wherein the ion-selectivemembrane forms a layer interposed in a propagation path of theillumination light along the waveguide, and a thickness of theion-selective membrane in the waveguide is larger than a coherencelength of the illumination light in free space.
 15. The device of claim13, wherein the ion-selective membrane forms a layer interposed in apropagation path of the illumination light along the waveguide, thelayer having a front and back surfaces in the illumination path, suchthat the front and back surfaces are not parallel to each other.